The present invention relates to electrical filters for passing a band of frequencies, and more particularly to surface acoustic wave filters, commonly referred to as SAW filters.
A typical SAW filter includes a flat piezoelectric wafer as a substrate on which are disposed an input transducer and an output transducer, each having a plurality of interdigital fingers. The spacing between the interdigital fingers of the transducers defines the wavelength of surface acoustic waves to which the transducer responds. The wavelength thus defined determines a passband center frequency for the respective transducer. When an input signal is coupled to the input transducer, a voltage is applied between adjacent interdigital fingers thereof which deforms the surface thereunder, resulting in an elastic wave that is propagated across the surface of the piezoelectric wafer to the output transducer. At the output transducer, the elastic deformation polarizes the surface of the wafer and sets up a voltage between the interdigital fingers thereof, provided that the interdigital fingers of the output transducer are properly spaced in relation to the polarization pattern induced in the wafer by the input transducer.
The interdigital fingers of the input and output transducers may be implemented with any suitable conventional type of weighting, such as, for example, apodized weighting where the overlap of adjacent interdigital fingers is sinewave contoured for improving the passband characteristics of the filter. The design and characteristics of SAW filters are generally described in the report entitled, "New Analog Surface Wave Filter Techniques and Applications", published by the National Electronics Conference in 1972.
SAW filters may also be arranged in a three-transducer configuration, where an input transducer is surrounded by two output transducers for increasing the overall filter gain. Various types of three-transducer SAW filters are described in U.S. Pat. Nos. 3,582,838, 3,582,840, 3,970,970, 4,044,321, 4,047,130 and 4,063,202. These prior art three-transducer SAW filters provide somewhat improved electrical characteristics over two-transducer SAW filters, but require larger wafers and more interdigital fingers due to the additional transducer. However, as the number of interdigital fingers increases, both filter distortion due to the mass loading and electrical discontinuities of each finger, and fabrication costs increase proportionally. Furthermore, it is often desirable to provide multiple passbands in a single SAW filter. In order to provide multiple-passband characteristics with prior art SAW filters, it has been necessary to utilize two or more of the foregoing two-transducer or three-transducer SAW filters and a summing circuit for combining the respective filter output signals to provide a composite output signal.